Studies will be done to identify structural and functional heterogeneity in murine monocyte populations. Subpopulations will be obtained with the use of plastic adherence and rosetting techniques in conjunction with hapten-modified anti-cell surface reagents. Subpopulations from normal tissues and several murine monocyte cell lines will be assayed for their ability to process and present antigen and for their ability to produce several immunoregulatory mediators and other macrophage products. The capacity of the various cells to respond to different inducing regimens and to respond to different inhibitors will be investigated.